NEW YORK (Reuters Health) - A common implantable device that helps stop dangerous heart rhythms by sending out small jolts of electricity sometimes misfires, Dutch researchers warn.

Over 5 years, they found more than one in six patients experienced the miscued jolts, which may not only frighten them, but could potentially also be dangerous.

Still, they stress the so-called implantable cardioverter-defibrillator, or ICD, is a proven lifesaver for heart attack survivors and others with high-risk heart disease.

And although people whose ICD misfired were more likely to die during the study, this doesn't prove the malfunction kills patients, according to the researchers, who have received funding from companies that make the devices.

ICDs resemble pacemakers, but are somewhat more complex. They monitor the heart constantly and are programmed to send shocks only when they detect dangerous disturbances in the heart's rhythm, called ventricular tachycardia (VT) or ventricular fibrillation (VF), which cause about a third of all deaths from heart disease.

A shock delivered in the absence of such disturbances is a "devastating event for patients and their relatives," Dr. Martin J. Schalij of Leiden University Medical Center told Reuters Health in an email.

"Not only is it painful, but it also comes unexpectedly. Furthermore, in some trials, inappropriate therapy is associated with increased mortality," he said.

To get a better handle on these risks in a general patient population, Schalij and his colleagues studied more than 1,500 patients who had had an ICD implanted at their medical center between 1996 and 2006.

The researchers found that 13 percent of their patients experienced at least one ICD misfire over 3.5 years on average. This risk rose as the study went along, from 7 percent of the patients after the first year up to 18 percent after 5 years.

The researchers calculated that with one misfire, a patient's risk of dying in the study rose by 60 percent. With five, it was up nearly four-fold.

Overall, 19 percent of the patients died during the course of the study.

A history of abnormal heart rhythms, or atrial fibrillation, doubled a patient's chances of experiencing an inappropriate shock. Patients under the age of 70 also had a greater risk than older patients.

No significant differences appeared in the rates of wrong signals sent by the various ICD brands used by patients.

The researchers did, however, find some differences over time -- and not in the direction they expected. Despite the recent improvements made in ICD technology, patients who had their device implanted between May 2004 and 2006 were actually at a greater risk of inappropriate shocks than those who got them earlier.

This surprising finding may be explained by "evolving guidelines" for ICDs, note the researchers, which could have resulted in more critically ill patients receiving them in later years.

Given that a misdiagnosis of the patient's heart rhythm disturbance was the main cause of inappropriate shocks, improving the ICDs; ability to discriminate between false alarms and serious problems is key, the researchers say.

"We should focus on preventing inappropriate shocks by patient-tailored programming," Schalij said, adding that despite its drawbacks, implanting an ICD is still the right treatment option for many heart patients.

SOURCE: http://bit.ly/elrge2 Journal of the American College of Cardiology, online January 24, 2010.